DEVELOPMENTAL STAGE: Not expressed in meristemoids, but strongly expressed in guard mother cells (GMCs) and in young guard cells (at protein level) (PubMed:17088607). Expressed at the transition to terminal stomatal differentiation, just before and after the symmetric division of stomatal differentiation, being confined to late-stage GMC and to young, still differentiating guard cells (PubMed:24571519). {ECO:0000269|PubMed:17088607, ECO:0000269|PubMed:24571519}.